Reduces visibility to keep the compatibility.

Adding public methods may break the subclasses if a method with
same signature defined with less visibility.

Also changes devmode version of ClientSerializationStreamReader
to use the server-side base64 implementation to workaround devmode
issue related to accessing package visibile method in supersourced
classes.

Change-Id: I2c928ea88a27e45b90ddc3f7efd237cbddad84ee

user/src/com/google/gwt/user/client/rpc/impl/AbstractSerializationStream.java

@@ -116,7 +116,7 @@ protected final void setVersion(int version) {
    *
    * Keep this synchronized with the version in Base64Utils.
    */
-  public static long longFromBase64(String value) {
+  static long longFromBase64(String value) {
     int pos = 0;
     long longVal = base64Value(value.charAt(pos++));
     int len = value.length();
@@ -133,7 +133,7 @@ public static long longFromBase64(String value) {
    *
    * Keep this synchronized with the version in Base64Utils.
    */
-  public static String longToBase64(long value) {
+  static String longToBase64(long value) {
     // Convert to ints early to avoid need for long ops
     int low = (int) (value & 0xffffffff);
     int high = (int) (value >> 32);

user/src/com/google/gwt/user/client/rpc/impl/ClientSerializationStreamReader.java

@@ -37,6 +37,7 @@
 import com.google.gwt.dev.js.ast.JsVisitor;
 import com.google.gwt.user.client.rpc.IncompatibleRemoteServiceException;
 import com.google.gwt.user.client.rpc.SerializationException;
+import com.google.gwt.user.server.Base64Utils;
 
 import java.io.StringReader;
 import java.util.ArrayList;
@@ -382,7 +383,8 @@ public int readInt() {
 
   @Override
   public long readLong() {    
-    return longFromBase64(((JsStringLiteral) decoder.getValues().get(--index)).getValue());
+    return Base64Utils.longFromBase64(
+        ((JsStringLiteral) decoder.getValues().get(--index)).getValue());
   }
 
   @Override